Electron Temperatures in the Dayside Ionosphere of Mars Derived from Chemistry

Abstract

From a reduced chemical model of the Martian dayside ionosphere we derive an expression that can be used to estimate the electron temperature as a function of the ambient number densities of CO 2 , O, , , O + , and NO + and the total ion number density. The model is tested in the sunlit ionosphere with Mars Atmosphere and Volatile EvolutioN /Neutral Gas Ion Mass Spectrometer ( MAVEN /NGIMS) data from the Deep Dip campaigns DD2 (2015 April) and DD8 (2017 October). Around an altitude of ∼130–140 km the calculated electron temperatures along the DD2 orbits are in many cases compatible with the neutral temperatures derived from the CO 2 density profiles and downward integration of the hydrostatic balance equation, indicating efficient cooling of the electron population. For altitudes below 170 km the electron temperature (constructed from median density profiles) are higher for DD8 than for DD2 which we link to atmospheric solar cycle modulation. Median electron temperatures derived for DD2 and DD8 are roughly similar when inspecting similar CO 2 number densities and a simple power law is proposed to relate the electron temperature to the CO 2 number density. Calculated electron temperatures of approximately 1000 K around an altitude of 180–200 km appear not to conflict with published data from MAVEN Langmuir Probe Wave (LPW) measurements. At greater depths the LPW-derived electron temperatures have been reported as biased high and so a detailed comparison with results from the present work is merely proposed as a dedicated follow-up study.